This application claims the benefits of and priority to Indian Patent Application No. 833/Del/99, filed Jun. 3, 1999, which status is pending, and is hereby incorporated by reference in its entirety.
1. Field of the Invention
This invention relates to compositions having niobate anions and a combination of lead, barium, lanthanum, and bismuth cations. These compositions are part of the Tungsten Bronze class of ferroelectric ceramic materials, and may have very high piezoelectric charge coefficients, d33, and high dielectric constants. The invention also relates to processes of preparing the compositions, and ferroelectric ceramic materials and articles comprising the composition.
2. Background Art
When barium is partly substituted for lead in lead metaniobate, there is morphotrophic phase boundary at barium concentrations of about 0.4. In fact Francombe has shown (Acta Cryst. 13, 131-40 (1960)) that for barium concentration of 0.375 to 0.475, a mixed phase of tetragonal and orthorhombic structure occur. Subbarao et al. reported extensive work on substituted lead metaniobate, (J. chem. Phys. 32, 1846-51 (1960)). Pioneering work on hot pressed lead barium lanthanum niobate (PBLN) was done by Nagata, Okazaki et al. In this ceramic, grains are needle shaped and oriented parallel to c-axis.
Baxter and Hellicar (J. Am Ceram Soc. 43, 578-83 (1960)) prepared lead barium niobates and studied their electrical properties. They found that the structure of PbxBa(1xe2x88x92x)Nb2O6 is orthorhombic for Barium content in the range 0-0.4 and the Curie temperature decreases for the same range of composition. With further increases of barium content, the structure become tetragonal and Curie temperature again rises. Mechanical Qm is 5 and 8 for barium contents between about 0 and 0.4, and as barium content increases, there is a sharp increase of Qm to 1000. There is a morphotrophic phase boundary between the orthorhombic and tetragonal structures at barium concentrations of about 0.4.
Substitution of barium for lead in PbNb2O6 decreases the orthorhombic distortion and then induces a tetragonal structure with polar axis along xe2x80x98cxe2x80x99 rather than orthorhombic xe2x80x98bxe2x80x99 (See Francombe, Acta Cryst. 13,131-40 (1960); Lewis and Thomas, Proc. Internet. Conf. Solid State Phys. Electronics Telecommun., Brussels 4, Pt. 2, 883-90 (1958, Publ. 1960); Isupov, and Kosiakov, Zh. Tekh. Fiz. 28, 2175-85 and Soviet Phys.xe2x80x94Tech. Phys. 3, 2002-10 (1958); Goodman, G,. Am.Ceram. Soc. Bull. 34, No. 4, Program 11 (1955); U.S. Pat. Nos. 2,805,165; and 2,729,757). The substitution of substantial amounts of barium in lead niobate, PbNb2.O6, causes remarkable change in ferro-electric properties. Greatly enhanced dielectric and piezoelectric properties are observed, as the barium content approaches 0.4.
During 1983, Nagata et al. studied the electric properties of hot-pressed (PbxBa1xe2x88x92x)1-3y/2LayNb2O6, (see Japanese Journal of Applied Physics, Vol. 22 Supplement 22-2, pp 123-125 (1983); and Japanese Journal of Applied Physics, Vol. 24 Supplement 24-3, pp 100-102 (1985)). They found that kt increases on tetragonal side up to x/y=60/4 and the maximum value of kt is 0.4. In the orthorhombic x/y=80/4 side kt decreases, the value of Q is 6, 12 and is low. Further, in the orthorhombic side, when x/y=65/4 and 70/4 remnant polarization Pr attains maximum value of 10.
Nagata et al, in 1985 prepared PBLN by molten salt synthesis and found the maximum value of kt=0.36 and lowest Qm=30 and they concluded that the needle shaped PBLN prepared by molten salt synthesis become smaller in size and shorter in length with increase in and Lanthanum substitution. They concluded that with introduction of Lanthanum grain orientation fades as length to diameter ratio of grains decreases.
Neurogoankar et.al. studied hot pressed samples of Pb0.6Ba0.4Nb2O6 with or without Lanthanum (See Mat. Res.Bull. Vol. 26, pp. 771-777, 1991). It was found that the value of Ps (spontaneous polarization)=is 10.9 xcexcc/cm2 in PBN: 60/6 and 23.6 xcexcc/cm2 in PBN. They reported d33 values of 275xc3x9710xe2x88x9212 C/N in PBLN 60/6 and 236xc3x9710xe2x88x9212 C/N in PBN 60.
In all previous works, whenever a dopant was substituted, it was substituted both in lead and barium sites. Poling of these ceramics caused problems due to a decrease of resistance at high temperature. SUMMARY OF THE INVENTION
The object of this invention is to prepare compositions and/or ceramic materials having strong piezoelectric and/or ferroelectric properties of the tungsten bronze type class of ceramic materials, which can be poled easily.
To achieve the said objective, in one embodiment the invention relates to compositions having Nb2O62xe2x88x92 anions (niobate anions) in combination with lead, barium, lanthanum, and bismuth cations.
In a preferred embodiment, the invention relates to compositions of the general formula Pb(xxe2x88x923y/2xe2x88x923z/2)Ba(1xe2x88x92x)LayBizNb2O6, wherein x is preferably from about 0.4 to about 0.6, y is preferably from about 0.01 to about 0.03 and z is preferably from about 0.01 to about 0.03.
The above-described compositions may be easily poled, to provide piezoelectric and/or ferroelectric ceramic materials, that preferably have high piezoelectric charge coefficients, i.e. high d33 values and/or high dielectric constants.
The instant invention further relates to processes for preparing ferroelectric ceramic materials of general formula Pb(xxe2x88x923y/2xe2x88x923z/2)Ba(1xe2x88x92x)LayBizNb2O6, wherein x is preferably from about 0.4 to about 0.6, y is preferably from about 0.01 to about 0.03 and z is preferably from about 0.01 to about 0.03, and articles comprising the ferroelectric materials.
In one embodiment, the invention relates to compositions having Nb2O62xe2x88x92 anions (niobate anions) in combination with lead, barium, lanthanum, and bismuth cations.
In a preferred embodiment, the invention relates to compositions of the general formula Pb(xxe2x88x923y/2xe2x88x923z/2)Ba(1xe2x88x92x)LayBizNb2O6, wherein x is preferably from about 0.4 to about 0.6, y is preferably from about 0.01 to about 0.03 and z is preferably from about 0.01 to about 0.03. In more preferred embodiments, x is about 0.6, y is about 0.02, and z is about 0.02. In other more preferred embodiments, x is about 0.6, y is about 0.02, and z is about 0.03. Preferred compositions of the invention also include Pb0.54Ba0.4La0.02Bi0.02Nb2O6 and Pb0.525Ba0.4La0.2Bi0.03Nb2O6.
The compositions of the invention may be poled, to provide piezoelectric and/or ferroelectric ceramic materials. Alternatively, the compositions may be formed into articles, then poled, to provide piezoelectric and/or ferroceramic articles. The ferroelectric ceramic materials and/or articles of the invention may be formed into articles that are surprisingly easily poled, i.e. the microscopic domains of the material and/or article may be re-oriented in a desired direction, preferably by applying a d.c. electric field to the material. The ferroelectric ceramic materials and/or articles exhibit unexpectedly high piezoelectric charge coefficients, i.e. high d33 values, that may be greater than about 300 pc/N.
The invention also provides processes for preparing the piezoelectric and/or ferroelectric ceramic materials and/or articles. The articles are useful as components of ultra-sonic transducers.
In one embodiment, the processes for preparing Pb(xxe2x88x923y/2xe2x88x923z/2)Ba(1xe2x88x92x)LayBizNb2O6 comprise doping PbxBa(1xe2x88x92x)Nb2O6 with Bi and La in place of lead only, wherein the barium sites are left undisturbed, leading to tightly packed structure resulting in an improved piezoelectric material with high d33 value.
In preferred embodiments of processes of the invention, the ferroelectric ceramic materials of the invention are prepared by calcining oxides or carbonates of Pb and Ba and oxides of La, Bi, and Nb in predetermined proportions, at high temperatures. The oxides of Pb, Ba, La, Bi, and/or Nb may include any oxide or carbonate compound containing one or more metals, salts or oxides comprising Pb, Ba, La, Bi, and/or Nb.
Preferably, the compositions and ferroelectric ceramic materials are prepared by calcining mixtures of metal oxide or carbonate compounds, wherein the oxide of Pb is PbO, the oxide of Ba BaO3, the oxide of La is La2O3, the oxide of Bi is Bi2O3and the oxide of Nb is Nb2O5. The carbonates of Pb is PbCO3 and the carbonate of Ba is BaCO3, and the carbonate of La is LaCO3, Preferably, the oxides or carbonates of Pb and Ba and oxides of La, Bi, and Nb employed in the processes are high purity compounds. Even more preferably, the oxides or carbonates of Pb and Ba and oxides of La, Bi, and Nb are analytical reagent grade materials.
The oxides or carbonates of Pb and Ba and oxides of La, Bi, and Nb are combined in predetermined molar proportions, so as to produce the desired compositions and/or ferroelectric ceramic materials of the invention, i.e. Pb(xxe2x88x923y/2xe2x88x923z/2)Ba(1xe2x88x92x)LayBizNb2O6. In these compositions, x is preferably from about 0.4 to about 0.6, y is preferably from about 0.01 to about 0.03, and z is preferably from about 0.01 to about 0.03. Preferably, the predetermined molar proportions of the oxides of Pb, Ba, La, Bi, and Nb are about the same as the molar proportions of Pb, Ba, La, Bi, and Nb in the product of the process, i.e., Pb(xxe2x88x923y/2xe2x88x923z/2)Ba(1xe2x88x92x)LayBizNb2O6. Nevertheless, it is to be understood that some variations in the proportions of the oxides of Pb, Ba, La, Bi, and Nb combined may be required in order to produce a final product of the desired composition. For example, at the high temperatures of calcining, PbO can be volatilized, and therefore lost from the product. Therefore, PbO may optionally be combined in the form of a 4-6% molar excess of PbO, to compensate for PbO evaporation during firing or calcining.
In preferred embodiments of the above-described processes, analytical reagent grade metal oxides are employed. In preferred embodiments, the oxides are dry ground for 4-6 hours before calcining.
Calcining comprises heating the oxides or carbonates of Pb and Ba and oxides of La, Bi, and Nb at a temperature and for a time sufficient to form the desired products. Preferably, the calcining occurs at a temperature of about 800-900xc2x0 C. Preferably, the calcining occurs over a time period from about 2 to about 6 hours.
The compositions and/or ferroelectric ceramic materials of the invention, produced by the above-described processes, may be processed to produce articles comprising the ferroelectric ceramic materials. Preferred processes for preparing the article comprises:
cooling the composition that results from the above-described calcining step and grinding it and mixing it with a binder to form pellets,
sintering the pellets in presence of air to form a sintered article,
applying a silver-containing composition to the surface of the sintered article and firing, and
poling the sintered article.
In preferred embodiments of processes for preparing the article, the grinding, of the composition is carried out in presence of an alcohol solvent, preferably methanol. The ground product is mixed with a binder to form pellets. Preferred binders comprise organic polymers. Polyvinyl alcohol is a preferred binder. The pellets are then shaped, molded, or compressed to form articles. The articles of the invention may be any shape that may be suitable for the intended application of the article, such as a disk.
Sintering is carried out at an elevated temperature, and for a time effective to form the sintered article. Preferably, the temperature of sintering is above about 1000xc2x0 C., and more preferably from about 1250 to about 1280xc2x0 C. The time required for effective sintering varies with composition, temperature, and other variables, but determining effective times requires only routine experimentation within the average level of skill in the art. Preferably, when the temperature is from about 1250 to about 1280xc2x0 C., the sintering time is from about 1 to about 1xc2xd hrs.
In preferred embodiments of the processes of the invention, analytical reagent grade oxides are calcined at 800xc2x0 C. for 6 hours and the pellets are sintered in air at 1280xc2x0 C., for 1 hour, to form discs.
The compositions, the ceramic materials, and the sintered articles may be poled, to re-orient the domains of the ceramic material, preferably by exposing the article to a d.c. electric field.
Preferably, poling is preceded by polishing the surfaces of the article, then applying a silver-containing composition to one or more surfaces of the article and firing, to coat the surface of the article with metallic silver. Preferably, the article is coated with the silver-containing composition on two or more sides. The silver-containing composition preferably comprises a paste containing silver oxide, which is at least partially converted to metallic silver on the surface of the article by firing and/or heating the article to temperatures of approximately 500xc2x0 C. for time periods of time effective to at least partially convert the silver oxide to metallic silver. Preferably, the article is heated at temperatures up to 500xc2x0 C. for 15-20 minutes.
A preferred method of poling is by corona discharge, employing an electric field. Preferably, the poling is conducted at 120-130xc2x0 C., using an electric field from about 35 to about 46 KV/cm, for from about 25 to about 35 min. In certain preferred embodiments, articles are poled by corona discharge at 125xc2x0 C. with an electric field of 40 KV/cm for 30 min.